THE Argument to the libretto, remarking that the story is ‘very diffusive’, attempts to summarise its principal elements. Some time in the past Teobaldo, general of Gustavo, King of the Cimbri, performed a Trovatore-type switch, exchanging his own son for Gustavo's son Sveno in the hope that his kin would inherit the throne. The pseudo-Sveno was later killed in battle by Faramondo, King of the Franks, on whom Gustavo swears an oath of ‘everlasting revenge’. Faramondo falls in love with Gustavo's daughter Rosimonda, who ‘tho’ she had a secret passion for him, endeavours his death, believing that he had killed her brother’. Gernando, King of the Svevi, his friend and ally, ‘who had conquered the Kingdom of the Cimbri for him’, also loves Rosimonda, and betrays Faramondo. Though the Argument does not mention it, Adolfo, another son of Gustavo, antagonises his father by falling in love with Faramondo's sister Clotilde, who returns his devotion, and Gustavo, presumably a middle-aged widower, also pursues Clotilde. No date or place is specified. The libretto does not draw the usual distinction between historical fact and fiction, no doubt because the history is bogus.

Act I opens in A cypress grove dedicated to vengeance, with an altar in the middle and preparations for a sacrifice. Gustavo, with Adolfo and soldiers in array, pours a vase of liquids on the altar and swears by the Elysian gods to wage perpetual war on Faramondo: whoever brings him his head, struck from his body, shall have Rosimonda and the Cimbrian throne. A short chorus calls for revenge. Teobaldo leads in to them Clotilde in chains. Gustavo condemns her to death, but a moment later stands in suspense and orders her release. The soldiers free her from her chains. She promises to be true to Adolfo provided he defends Faramondo. Adolfo begs forgiveness of Sveno's spirit for loving his enemy's sister; but love is all-powerful. The set changes to A Court in Rosimonda's royal palace. Childerico (the unrecognised Sveno) comes forth on one side, defending himself against Pharamond's soldiers; from the other side, Pharamond, with attendants, and Rosamond apart. Childerico is apparently Rosimonda's bodyguard. Faramondo stops the fight, identifies himself and lets Childerico go.